1. Field of the Invention
The present invention relates to a transistor output circuit for driving an output device according to an input signal, a semiconductor device having the transistor output circuit and a switching electric power unit having the transistor output circuit.
2. Description of the Related Art
Various types of switching electric power units are conventionally used for electronic devices, which require another voltage except for power-supply voltage, such as digital cameras, PC, PDA, cellular phones, motor drivers and so forth.
FIG. 5 is a view showing a conventional example of the constitution of a main circuit section 20 of a switching electric power unit and a transistor output circuit (driver) 30 for driving the main circuit section 20. FIG. 6 is timing chart of drive signals.
In this switching electric power unit, a high voltage can be obtained when energy is accumulated in and discharged from a inductor Lo by turning on and off a control transistor Qo. The obtained high voltage is rectified by a diode Do and smoothed by a capacitor Co, so that DC output voltage Vo can be obtained.
For example, an input signal Sin such as PWM control signal is inputted into the driver 30, and an output signal Sout for turning on and off the control transistor Qo is outputted from the driver 30. The driver 30 includes a CMOS (Complementary MOS) circuit, in which a P-type MOSFET (hereinafter, PMOS) Q1 and an N-type MOSFET (hereinafter, NMOS) Q2 are directly connected to output the output signal Sout, a drive signal generating circuit 31 for turning on and off the CMOS circuit according to the input signal Sin, and inverters INV 1 to INV 4.
In order to eliminate or reduce a short circuit current generated at the time of changing between ON and OFF in CMOS circuit, it is necessary to shift a generation timing of a positive side drive signal Pg for driving the PMOS Q1 and a generation timing of a negative side drive signal Ng for driving the NMOS Q2. Therefore, as shown in FIG. 6, the width of the positive side drive signal Pg is extended to off-delay with respect to the width of the input signal Sin, and the width of the negative side drive signal Ng is reduced to on-delay, so that off-period Td while both the PMOS Q1 and the NMOS Q2 are turned off is formed so as to reduce the short circuit current.
Although it is not a driver of the switching electric power unit, there is provided a compound circuit in which an NPN-type bipolar transistor for pull-up (hereinafter, NPN transistor) and a PNP-type bipolar transistor for pull-down (hereinafter, PNP transistor) are connected to the electric power unit in series and these NPN and PNP transistors are driven by CMOS circuit (for example, JP-B-3026840).
Japanese Patent No.3028840 (Right column of page 3, FIG. 30) is known as related reference.
However, in the case where the frequency of the input signal Sin is low (for example, about 500 kHz), even when a sufficiently long off-period Td is provided like the conventional case shown in FIG. 5, problems are seldom caused. However, operating frequency of a switching electric power unit has been recently increased according to the demand of reducing the sizes of parts to be used. When the off-period Td is shortened in order to meet the demand, an intensity of the short circuit current is increased. Further, an output voltage of CMOS circuit is suddenly varied to an upper and a lower limit in both cases of rising and falling, and the switching wave-form becomes a substantial square wave-form. Due to the increase in the short circuit current and the square switching wave-form, electromagnetic noise increases in respect of EMI (ElectroMagnetic Interference).
In the circuit described in Japanese Patent No.3028840, a short circuit current, the intensity of which corresponds to the driving capacity, flows in CMOS circuit to drive NPN and PNP. The output signal Sout to be outputted is limited by voltage Vf between base and emitter (about 0.6 to 0.7 [V]) of NPN transistor and PNP transistor. The voltage of the output signal Sout rises to only a potential that is lower than the power-supply voltage by voltage Vf. In the same manner, the output signal Sout falls to only a potential higher than the power-supply voltage by voltage Vf.
Therefore, a transistor switch driven by output signal Sout can not be completely turned off, so that a leakage current flows to generate a loss. Further, there is a possibility that the performance of a load device (for example, the main circuit of the switching electric power unit) is deteriorated. Furthermore, as a switching wave-form of the output voltage is also a square wave-form, electromagnetic noise increases.
The object of the present invention is to provide a transistor output circuit that enables to reduce electromagnetic noise and provides a switching output signal which has a sufficiently large amplitude by remarkably reducing the short circuit current, increasing/ decreasing an output signal to the power-supply potential/the ground potential, and smoothly changing in the proximity of the power-supply potential and the ground potential,.
Another object of the present invention is to provide a semiconductor device including the transistor output circuit and a switching electric power unit provided with the transistor output circuit.
The present invention provides a transistor output circuit having: a pre-driving circuit having a first CMOS circuit driven by an input signal, which is connected between an electric power unit and a ground; a main driving circuit having an NPN-type transistor and a PNP-type transistor driven by an output of the pre-driving circuit, which are connected between the electric power unit and the ground in series, and outputting an output signal from a series connection point of the NPN-type transistor and the PNP-type transistor; and an auxiliary driving circuit having a second CMOS circuit driven by the input signal and a current limit resistor for limiting an output current of the second CMOS circuit, which are connected between the electric power unit and the ground, wherein an output point of the auxiliary driving circuit is connected to the series connection point.
Furthermore, the pre-driving circuit includes a first resistor, a second resistor, and the first CMOS circuit having a PMOS and a NMOS, in which the PMOS, the first resistor, the second resistor, and the NMOS are connected in this order between the electric power unit and the ground in series, and outputs from a connection point of the first resistor and the second resistor, and the current limit resistor of the auxiliary driving circuit includes a first current limit resistor and a second current limit resistor, the second CMOS circuit includes a PMOS and an NMOS , in which the PMOS, the first current limit resistor, the second current limit resistor, and the NMOS are connected in this order between the electric power unit and the ground in series, and the auxiliary driving circuit outputs from a connection point of the first current limit resistor and the second current limit resistor. Furthermore, at least one of the first current limit resistor and the second current limit resistor has a plurality of resistor elements in series and/or parallel.
Furthermore, a control transistor controlled to be turned on and off by the output signal.
The present invention provides a semiconductor integrated circuit having the transistor output circuit.
The present invention provides a switching electric power unit having: a transistor output circuit including a pre-driving circuit having a first CMOS circuit driven by an input signal, which is connected between an electric power unit and a ground; a main driving circuit having an NPN-type transistor and a PNP-type transistor driven by an output of the pre-driving circuit, which are connected between the electric power unit and the ground in series, and outputting an output signal being outputted from the a series connection point of the NPN-type transistor and the PNP-type transistor; and an auxiliary driving circuit having a second CMOS circuit driven by the input signal and also having a current limit resistor for limiting an output current of the second CMOS circuit, which are connected between the electric power unit and the ground, wherein an output point of the auxiliary driving circuit is being connected to the series connection point; and a main circuit section, which includes a control transistor controlled to be turned on and off by the output signal from the transistor output circuit, outputting an output signal which voltage is converted from a voltage of the electric power unit according to the operation of turning on and off of the control transistor.
Furthermore, the pre-driving circuit includes a first resistor, a second resistor, and the first CMOS circuit having a PMOS and a NMOS, in which the PMOS, the first resistor, the second resistor, and the NMOS are connected in this order between the electric power unit and the ground in series, and outputs from a connection point of the first resistor and the second resistor, and the current limit resistor of the auxiliary driving circuit includes a first current limit resistor and a second current limit resistor, the second CMOS circuit includes a PMOS and an NMOS, in which the PMOS, the first current limit resistor, the second current limit resistor, and the NMOS are connected in this order between the electric power unit and the ground in series, and the auxiliary driving circuit outputs from a connection point of the first current limit resistor and the second current limit resistor.